Increasingly severe automobile regulations in respect of emissions and fuel efficiency are placing increasing demands on both engine manufacturers and lubricant formulators to provide effective solutions to improve fuel economy.
Optimising lubricants through the use of high performance basestocks and novel additives represents a flexible solution to a growing challenge.
Friction-reducing additives (which are also known as friction modifiers) are important lubricant components in reducing fuel consumption and various such additives are already known in the art. Friction modifiers can be conveniently divided into two categories, that is to say, metal-containing friction modifiers and ashless (organic) friction modifiers.
Organo-molybdenum compounds are among the most common metal-containing friction modifiers. Typical organo-molybdenum compounds include molybdenum dithiocarbamates (MoDTC), molybdenum dithiophosphates (MoDTP), molybdenum amines, molybdenum alcoholates, and molybdenum alcohol-amides. WO-A-98/26030, WO-A-99/31113, WO-A-99/47629 and WO-A-99/66013 describe tri-nuclear molybdenum compounds for use in lubricating oil compositions.
However, the trend towards low-ash lubricating oil compositions has resulted in an increased drive to achieve low friction and improved fuel economy using ashless (organic) friction modifiers.
Ashless (organic) friction modifiers typically comprise esters of fatty acids and polyhydric alcohols, fatty acid amides, amines derived from fatty acids and organic dithiocarbamate or dithiophosphate compounds. Further improvements in lubricant performance characteristics have been achieved through the use of synergistic behaviours of particular combinations of lubricant additives.
WO-A-99/50377 discloses a lubricating oil composition which is said to have a significant increase in fuel economy due to the use therein of tri-nuclear molybdenum compounds in conjunction with oil soluble dithiocarbamates.
EP-A-1041135 discloses the use of succinimide dispersants in conjunction with molybdenum dialkyldithiocarbamates to give improved friction reduction in diesel engines.
U.S. Pat. No. 6,562,765 discloses a lubricating oil composition which is said to have a synergy between an oxymolybdenum nitrogen dispersant complex and an oxymolybdenum dithiocarbamate which leads to unexpectedly low friction coefficients.
EP-A-1367116, EP-A-0799883, EP-A-0747464, U.S. Pat. No. 3,933,659 and EP-A-335701 disclose lubricating oil compositions comprising various combinations of ashless friction modifiers.
WO-A-92/02602 describes lubricating oil compositions for internal combustion engines which comprise a blend of ashless friction modifiers which are said to have a synergistic effect on fuel economy.
The blend disclosed in WO-A-92/02602 is a combination of (a) an amine/amide friction modifier prepared by reacting one or more acids with one or more polyamines and (b) an ester/alcohol friction modifier prepared by reacting one or more acids with one or more polyols.
U.S. Pat. No. 5,114,603 and U.S. Pat. No. 4,683,069 describe lubricating oil compositions comprising mixtures of glycerol monooleate and glycerol dioleate in combination with other additives which were added for their conventional purpose.
EP-A-0747464 describes a lubricating oil composition for automatic transmissions which comprises alkoxylated fatty amines as well as a mixture of two friction modifiers which are selected from a large list of possible compounds. While said list includes glycerol esters, it is of note that there are no examples in EP-A-0747464 which comprise glycerol esters as friction modifiers.
U.S. Pat. No. 5,286,394 discloses a friction-reducing lubricating oil composition and a method for reducing the fuel consumption of an internal combustion engine. The lubricating oil composition disclosed therein comprises a major amount of an oil having lubricating viscosity and a minor amount of a friction-modifying, polar and surface active organic compound selected from a long list of compounds including mono- and higher esters of polyols and aliphatic amides. Glycerol monooleate and oleamide (i.e. oleylamide) are mentioned as examples of such compounds.
However, current strategies with regard to friction reduction for fuel economy oils are not sufficient to meet ever-increasing fuel economy targets set by Original Equipment Manufacturers (OEMs).
For example, molybdenum friction modifiers typically outperform ashless friction modifiers in the boundary regime and there is a challenge to approach similar levels of friction modification using solely ashless friction modifiers.
Thus, given the increasing fuel economy demands placed on engines, there remains a need to further improve the friction reduction and fuel economy of internal combustion engines utilising low ash lubricating oil compositions. It is therefore desirable to further improve on the performance of known ashless friction modifiers and known combinations of ashless friction modifiers, in particular to further improve on the friction-reducing performance of polyol ester friction modifiers such as glycerol monooleate that have been commonly used in the art.